Filter

ABSTRACT

A filter for filtering particulate matter (PM) from exhaust gas emitted from a positive ignition engine, which filter comprising a porous substrate having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a washcoat comprising a plurality of solid particles wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, and wherein the second mean pore size is less than the first mean pore size.

FIELD OF THE INVENTION

The present invention relates to a filter for use in treatingparticulate matter (PM) in exhaust gas derived from any combustionprocess where it is not possible to remove PM from the exhaust gas bybuild-up of PM (so-called “cake filtration”) or by a combination ofdepth filtration and cake filtration. The combustion process istypically that of a vehicular engine. In particular, the inventionrelates to a filter for use in treating PM derived from a vehicularpositive ignition engine, particularly stoichiometrically operatedpositive ignition engines but also lean-burn positive ignition engines.

BACKGROUND OF THE INVENTION

Positive ignition engines cause combustion of a hydrocarbon and airmixture using spark ignition. Contrastingly, compression ignitionengines cause combustion of a hydrocarbon by injecting the hydrocarboninto compressed air. Positive ignition engines can be fuelled bygasoline fuel, gasoline fuel blended with oxygenates including methanoland/or ethanol, liquid petroleum gas or compressed natural gas.

Ambient PM is divided by most authors into the following categoriesbased on their aerodynamic diameter (the aerodynamic diameter is definedas the diameter of a 1 g/cm³ density sphere of the same settlingvelocity in air as the measured particle):

(i) PM-10—particles of an aerodynamic diameter of less than 10 μm;

(ii) Fine particles of diameters below 2.5 μm (PM-2.5);

(iii) Ultrafine particles of diameters below 0.1 μm (or 100 nm); and

(iv) Nanoparticles, characterised by diameters of less than 50 nm.

Since the mid-1990's, particle size distributions of particulatesexhausted from internal combustion engines have received increasingattention due to possible adverse health effects of fine and ultrafineparticles. Concentrations of PM-10 particulates in ambient air areregulated by law in the USA. A new, additional ambient air qualitystandard for PM-2.5 was introduced in the USA in 1997 as a result ofhealth studies that indicated a strong correlation between humanmortality and the concentration of fine particles below 2.5

Interest has now shifted towards nanoparticles generated by diesel andgasoline engines because they are understood to penetrate more deeplyinto human lungs than particulates of greater size and consequently theyare believed to be more harmful than larger particles, extrapolated fromthe findings of studies into particulates in the 2.5-10.0 μM range.

Size distributions of diesel particulates have a well-establishedbimodal character that correspond to the particle nucleation andagglomeration mechanisms, with the corresponding particle types referredto as the nuclei mode and the accumulation mode respectively (see FIG.1). As can be seen from FIG. 1, in the nuclei mode, diesel PM iscomposed of numerous small particles holding very little mass. Nearlyall diesel particulates have sizes of significantly less than 1 μm, i.e.they comprise a mixture of fine, i.e. falling under the 1997 US law,ultrafine and nanoparticles.

Nuclei mode particles are believed to be composed mostly of volatilecondensates (hydrocarbons, sulfuric acid, nitric acid etc.) and containlittle solid material, such as ash and carbon. Accumulation modeparticles are understood to comprise solids (carbon, metallic ash etc.)intermixed with condensates and adsorbed material (heavy hydrocarbons,sulfur species, nitrogen oxide derivatives etc.) Coarse mode particlesare not believed to be generated in the diesel combustion process andmay be formed through mechanisms such as deposition and subsequentre-entrainment of particulate material from the walls of an enginecylinder, exhaust system, or the particulate sampling system. Therelationship between these modes is shown in FIG. 1.

The composition of nucleating particles may change with engine operatingconditions, environmental condition (particularly temperature andhumidity), dilution and sampling system conditions. Laboratory work andtheory have shown that most of the nuclei mode formation and growthoccur in the low dilution ratio range. In this range, gas to particleconversion of volatile particle precursors, like heavy hydrocarbons andsulfuric acid, leads to simultaneous nucleation and growth of the nucleimode and adsorption onto existing particles in the accumulation mode.Laboratory tests (see e.g. SAE 980525 and SAE 2001-01-0201) have shownthat nuclei mode formation increases strongly with decreasing airdilution temperature but there is conflicting evidence on whetherhumidity has an influence.

Generally, low temperature, low dilution ratios, high humidity and longresidence times favour nanoparticles formation and growth. Studies haveshown that nanoparticles consist mainly of volatile material like heavyhydrocarbons and sulfuric acid with evidence of solid fraction only atvery high loads.

Contrastingly, engine-out size distributions of gasoline particulates insteady state operation show a unimodal distribution with a peak of about60-80 nm (see e.g. FIG. 4 in SAE 1999-01-3530). By comparison withdiesel size distribution, gasoline PM is predominantly ultrafine withnegligible accumulation and coarse mode.

Particulate collection of diesel particulates in a diesel particulatefilter is based on the principle of separating gas-borne particulatesfrom the gas phase using a porous barrier. Diesel filters can be definedas deep-bed filters and/or surface-type filters. In deep-bed filters,the mean pore size of filter media is bigger than the mean diameter ofcollected particles. The particles are deposited on the media through acombination of depth filtration mechanisms, including diffusionaldeposition (Brownian motion), inertial deposition (impaction) andflow-line interception (Brownian motion or inertia).

In surface-type filters, the pore diameter of the filter media is lessthan the diameter of the PM, so PM is separated by sieving. Separationis done by a build-up of collected diesel PM itself, which build-up iscommonly referred to as “filtration cake” and the process as “cakefiltration”.

It is understood that diesel particulate filters, such as ceramicwallflow monoliths, may work through a combination of depth and surfacefiltration: a filtration cake develops at higher soot loads when thedepth filtration capacity is saturated and a particulate layer startscovering the filtration surface. Depth filtration is characterized bysomewhat lower filtration efficiency and lower pressure drop than thecake filtration.

WO 03/011437 discloses a gasoline engine having an exhaust systemcomprising means for trapping PM from the exhaust gas and a catalyst forcatalysing the oxidation of the PM by carbon dioxide and/or water in theexhaust gas, which catalyst comprising a supported alkali metal. Themeans for trapping PM is suitable for trapping PM of particle range10-100 nm, and can be a wallflow filter made from a ceramic material ofappropriate pore size such as cordierite coated with the catalyst, ametal oxide foam supporting the catalyst, a wire mesh, a diesel wallflowfilter designed for diesel applications, an electrophoretic trap or athermophoretic trap (see e.g. GB-A-2350804).

WO 2008/136232 A1 discloses a honeycomb filter having a cell wallcomposed of a porous cell wall base material and, provided on its inflowside only or on its inflow and outflow sides, a surface layer andsatisfying the following requirements (1) to (5) is used as a dieselparticulate filter: (1) the peak pore diameter of the surface layer isidentical with or smaller than the average pore diameter of the cellwall base material, and the porosity of the surface layer is larger thanthat of the cell wall base material; (2) with respect to the surfacelayer, the peak pore diameter is from 0.3 to less than 20 μm, and theporosity is from 60 to less than 95% (measured by mercury penetrationmethod); (3) the thickness (L1) of the surface layer is from 0.5 to lessthan 30% of the thickness (L2) of the cell wall; (4) the mass of thesurface layer per filtration area is from 0.01 to less than 6 mg/cm²;and (5) with respect to the cell wall base material, the average porediameter is from 10 to less than 60 μm, and the porosity is from 40 toless than 65%. See also SAE paper no. 2009-01-0292.

Other techniques suggested in the art for separating gasoline PM fromthe gas phase include vortex recovery.

Emission legislation in Europe from 1 Sep. 2014 (Euro 6) requirescontrol of the number of particles emitted from both diesel and gasoline(positive ignition) passenger cars. For gasoline EU light duty vehiclesthe allowable limits are: 1000 mg/km carbon monoxide; 60 mg/km nitrogenoxides (NO_(x)); 100 mg/km total hydrocarbons (of which ≦68 mg/km arenon-methane hydrocarbons); and 4.5 mg/km particulate matter ((PM) fordirect injection engines only). Although the authorities have not setthe PM number standard for Euro 6 yet, it is widely understood that itwill be set at 6.0×10¹¹ per km. The present specification is based onthe assumption that this number will be adopted in due course.

In the United States, no similar emission standards have been set.However, the State of California Air Resources Board (CARB) recentlypublished a paper entitled “Preliminary Discussion Paper—Amendments toCalifornia's Low-Emission Vehicle [LEV] Regulations for CriteriaPollutants—LEV III” (release date 8 Feb. 2010) in which a new PMstandard of between 2 and 4 mg PM/mile (1.25-2.50 mg PM/km (currently 10mg PM/mile (6.25 mg PM/km))) is proposed, the paper commenting that:“Staff has received input from a number of manufacturers suggesting thata standard of 3 mg PM/mile (1.88 mg PM/km) can be met for gasolinedirect injection engines without requiring the use of particulatefilters.” Additionally, the paper states that since the PM mass andcount emissions appear to be correlated: “Although a mandatory numberstandard is not being considered at this time, an optional PM numberstandard of about 10¹² particles/mile [6.25¹¹ particles/km] is beingconsidered (which could be chosen by manufacturers instead of the PMmass standard)”. However, since neither the PM standard nor the PMnumber standard has been set by CARB yet, it is too soon to know whetherparticulate filtration will be necessary for the Californian or USvehicle market generally. It is nevertheless possible that certainvehicle manufacturers will choose filters in order to provide a marginof safety on any positive ignition engine design options selected tomeet whatever standards are eventually set.

The new Euro 6 emission standard presents a number of challenging designproblems for meeting gasoline emission standards. In particular, how todesign a filter, or an exhaust system including a filter, for reducingthe number of PM gasoline (positive ignition) emissions, yet at the sametime meeting the emission standards for non-PM pollutants such as one ormore of oxides of nitrogen (NO_(x)), carbon monoxide (CO) and unburnedhydrocarbons (HC), all at an acceptable back pressure, e.g. as measuredby maximum on-cycle backpressure on the EU drive cycle.

It is envisaged that a minimum of particle reduction for a three-waycatalysed particulate filter to meet the Euro 6 PM number standardrelative to an equivalent flowthrough catalyst is ≧50%. Additionally,while some backpressure increase for a three-way catalysed wallflowfilter relative to an equivalent flowthrough catalyst is inevitable, inour experience peak backpressure over the MVEG-B drive cycle (averageover three tests from “fresh”) for a majority of passenger vehiclesshould be limited to <200 mbar, such as <180 mbar, <150 mbar andpreferably <120 mbar e.g. <100 mbar.

PM generated by positive ignition engines has a significantly higherproportion of ultrafine, with negligible accumulation and coarse modecompared with that produced by diesel (compression ignition) engines,and this presents challenges to removing it from positive ignitionengine exhaust gas in order to prevent its emission to atmosphere. Inparticular, since a majority of PM derived from a positive ignitionengine is relatively small compared with the size distribution fordiesel PM, it is not practically possible to use a filter substrate thatpromotes positive ignition PM surface-type cake filtration because therelatively low mean pore size of the filter substrate that would berequired would produce impractically high backpressure in the system.

Furthermore, generally it is not possible to use a conventional wallflowfilter, designed for trapping diesel PM, for promoting surface-typefiltration of PM from a positive ignition engine in order to meetrelevant emission standards because there is generally less PM inpositive ignition exhaust gas, so formation of a soot cake is lesslikely; and positive ignition exhaust gas temperatures are generallyhigher, which can lead to faster removal of PM by oxidation, thuspreventing increased PM removal by cake filtration. Depth filtration ofpositive ignition PM in a conventional diesel wallflow filter is alsodifficult because the PM is significantly smaller than the pore size ofthe filter medium. Hence, in normal operation, an uncoated conventionaldiesel wallflow filter will have a lower filtration efficiency when usedwith a positive ignition engine than a compression ignition engine.

Another difficulty is combining filtration efficiency with a washcoatloading, e.g. of catalyst for meeting emission standards for non-PMpollutants, at acceptable backpressures. Diesel wallflow particulatefilters in commercially available vehicles today have a mean pore sizeof about 13 μm. However, we have found that washcoating a filter of thistype at a sufficient catalyst loading such as is described in US2006/0133969 to achieve required gasoline (positive ignition) emissionstandards can cause unacceptable backpressure.

In order to reduce filter backpressure it is possible to reduce thelength of the substrate. However, there is a finite level below whichthe backpressure increases as the filter length is reduced. Suitablefilter lengths for filters according to the present invention are from2-12 inches long, preferably 3-6 inches long. Cross sections can becircular and in our development work we have used 4.66 and 5.66 inchdiameter filters. However, cross-section can also be dictated by spaceon a vehicle into which the filter is required to fit. So for filterslocated in the so-called close coupled position, e.g. within 50 cm ofthe engine exhaust manifold where space is at a premium, elliptical oroval filter cross sections can be contemplated. As would be expected,backpressure also increases with washcoat loading and soot loading.

There have been a number of recent efforts to combine three-waycatalysts with filters for meeting the Euro 6 emission standards.

US 2009/0193796 discloses a three-way conversion catalyst coated onto aparticulate trap. The Examples disclose e.g. a soot filter having acatalytic material prepared using two coats: an inlet coat and an outletcoat. The mean pore size of the soot filter substrate used is notmentioned. The inlet coat contains alumina, an oxygen storage component(OSC) and rhodium all at a total loading of 0.17 g in⁻³; the outlet coatincludes alumina, an OSC and palladium, all at a total loading of 0.42 gin⁻³. However, we believe that the three-way catalyst washcoat loadingof <0.5 g in⁻³ provides insufficient three-way activity to meet therequired emission standards alone, i.e. the claimed filter appears to bedesigned for inclusion in a system for location downstream of athree-way catalyst comprising a flowthrough substrate monolith.

WO 2009/043390 discloses a catalytically active particulate filtercomprising a filter element and a catalytically active coating composedof two layers. The first layer is in contact with the in-flowing exhaustgas while the second layer is in contact with the out-flowing exhaustgas. Both layers contain aluminium oxide. The first layer containspalladium, the second layer contains an oxygen-storing mixedcerium/zirconium oxide in addition to rhodium. In Examples, a wallflowfilter substrate of unspecified mean pore size is coated with a firstlayer at a loading of approximately 31 g/l and a second layer at aloading of approximately 30 g/l. That is, the washcoat loading is lessthan 1.00 g in⁻³. For a majority of vehicle applications, this coatedfilter is unlikely to be able to meet the required emission standardsalone.

SUMMARY OF THE INVENTION

We have now discovered, very surprisingly, that it is possible to adapta relatively porous particulate filter—such as a particulate filteradapted for a diesel application—so that it can be used to trap e.g.ultrafine positive ignition PM at an acceptable pressure drop andbackpressure. In particular, our inventors have determined that awashcoat that hinders access of the PM to a porous structure of a filtersubstrate can beneficially promote surface filtration substantially atthe expense of depth filtration to the extent that cake filtration of PMderived from a positive ignition engine is promoted or enhanced.

Early indications suggest that positive ignition PM combusts in oxygenat lower temperatures than diesel PM. Investigations are continuing, butthe invention makes use of this observation by providing means fortrapping the positive ignition PM for combustion in oxygen.

According to one aspect, the invention provides a filter for filteringparticulate matter (PM) from exhaust gas emitted from an engine, such asa positive ignition engine, e.g. a vehicular positive ignition enginesuch as a stoichiometrically-operated positive ignition engine or a leanburn positive ignition engine, which filter comprising a poroussubstrate having inlet surfaces and outlet surfaces, wherein the inletsurfaces are separated from the outlet surfaces by a porous structurecontaining pores, e.g. surface pores, of a first mean pore size, whereinthe porous substrate is coated with a washcoat comprising a plurality ofsolid particles wherein the porous structure of the washcoated poroussubstrate contains pores of a second mean pore size, and wherein thesecond mean pore size is less than the first mean pore size.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood, reference ismade to the accompanying drawings wherein:

FIG. 1 is a graph showing the size distributions of PM in the exhaustgas of a diesel engine. For comparison, a gasoline size distribution isshown at FIG. 4 of SAE 1999-01-3530;

FIGS. 2A-C show schematic drawings of three embodiments of washcoatedporous filter substrates according to the invention;

FIG. 3 is a schematic graph of mercury porosimetry relating the poresize distribution of a porous filter substrate, a porous washcoat layerand a porous filter substrate including a porous surface washcoat layer;and

FIG. 4 is a Table setting out a matrix of wallflow filter substrate poresize vs. washcoat loading indicating the suitability of the coatedwallflow filter for use in a vehicular gasoline exhaust gasaftertreatment system.

DETAILED DESCRIPTION OF THE INVENTION

Early indications are that the present invention is capable of reducingpositive ignition engine particle number emissions by >30% such as >50%e.g. >80% or even >90% at acceptable backpressure.

Mean pore size can be determined by mercury porosimetry.

It will be understood that the benefit of the invention is substantiallyindependent of the porosity of the substrate. Porosity is a measure ofthe percentage of void space in a porous substrate and is related tobackpressure in an exhaust system: generally, the lower the porosity,the higher the backpressure. However, the porosity of filters for use inthe present invention are typically >40% or >50% and porosities of45-75% such as 50-65% or 55-60% can be used with advantage. The meanpore size of the washcoated porous substrate is important forfiltration. So, it is possible to have a porous substrate of relativelyhigh porosity that is a poor filter because the mean pore size is alsorelatively high.

The porous substrate can be a metal, such as a sintered metal, or aceramic, e.g. silicon carbide, cordierite, aluminium nitride, siliconnitride, aluminium titanate, alumina, cordierite, mullite e.g., acicularmullite (see e.g. WO 01/16050), pollucite, a thermet such as Al₂O₃/Fe,Al₂O₃/N₁ or B₄C/Fe, or composites comprising segments of any two or morethereof. In a preferred embodiment, the filter is a wallflow filtercomprising a ceramic porous filter substrate having a plurality of inletchannels and a plurality of outlet channels, wherein each inlet channeland each outlet channel is defined in part by a ceramic wall of porousstructure, wherein each inlet channel is separated from an outletchannel by a ceramic wall of porous structure. This filter arrangementis also disclosed in SAE 810114, and reference can be made to thisdocument for further details. Alternatively, the filter can be a foam,or a so-called partial filter, such as those disclosed in EP 1057519 orWO 01/080978.

Reasons motivating the coating of a wallflow filter for a dieselapplication are typically different from that of the present invention.In diesel applications, a washcoat is employed to introduce catalyticcomponents to the filter substrate, e.g. catalysts for oxidising NO toNO₂, yet a significant problem is to avoid backpressure issues as sootis accumulated. Accordingly, a balance is struck between the desiredcatalytic activity and acceptable backpressure. Contrastingly, a primarymotivating factor for washcoating a porous substrate for use in thepresent invention is to achieve both a desired filtration efficiency andcatalytic activity.

In one embodiment, the first mean pore size e.g. of surface pores of theporous structure of the porous filter substrate is from 8 to 45 μm, forexample 8 to 25 μm, 10 to 20 μm or 10 to 15 μm. In particularembodiments, the first mean pore size is >18 μm such as from 15 to 45μm, 20 to 45 μm e.g. 20 to 30 μm, or 25 to 45 μm.

In embodiments, the filter has a washcoat loading of >0.25 g in⁻³, suchas >0.5 g in⁻³ or ≧0.80 g in⁻³, e.g. 0.80 to 3.00 g in⁻³. In preferredembodiments, the washcoat loading is >1.00 g in⁻³ such as ≧1.2 gin⁻³, >1.5 g in⁻³, >1.6 g in⁻³ or >2.00 g in⁻³ or for example 1.6 to 2.4g in⁻³. In particular combinations of filter mean pore size and washcoatloading the filter combines a desirable level of particulate filtrationand catalytic activity at acceptable backpressure.

In a first, preferred embodiment, the filter comprises a surfacewashcoat, wherein a washcoat layer substantially covers surface pores ofthe porous structure and the pores of the washcoated porous substrateare defined in part by spaces between the particles (interparticlepores) in the washcoat. That is, substantially no washcoat enters theporous structure of the porous substrate. Methods of making surfacecoated porous filter substrates include introducing a polymer, e.g. polyvinyl alcohol (PVA), into the porous structure, applying a washcoat tothe porous filter substrate including the polymer and drying, thencalcining the coated substrate to burn out the polymer. A schematicrepresentation of the first embodiment is shown in FIG. 2A.

Methods of coating porous filter substrates are known to the skilledperson and include, without limitation, the method disclosed in WO99/47260, i.e. a method of coating a monolithic support, comprising thesteps of (a) locating a containment means on top of a support, (b)dosing a pre-determined quantity of a liquid component into saidcontainment means, either in the order (a) then (b) or (b) then (a), and(c) by applying pressure or vacuum, drawing said liquid component intoat least a portion of the support, and retaining substantially all ofsaid quantity within the support. Such process steps can be repeatedfrom another end of the monolithic support following drying of the firstcoating with optional firing/calcination.

In this first embodiment, an average interparticle pore size of theporous washcoat is 5.0 nm to 5.0 μm, such as 0.1-1.0 μm.

A D90 of solid washcoat particles in this first, surface coatingembodiment can be greater than the mean pore size of the porous filtersubstrate and can be in the range 10 to 40 μm, such as 15 to 30 μm or 12to 25 μm. “D90” as used herein defines the particle size distribution ina washcoat wherein 90% of the particles present have a diameter withinthe range specified. Alternatively, in embodiments, the mean size of thesolid washcoat particles is in the range 1 to 20 μm. It will beunderstood that the broader the range of particle sizes in the washcoat,the more likely that washcoat may enter the porous structure of theporous substrate. The term “substantially no washcoat enters the porousstructure of the substrate” should therefore be interpreted accordingly.

According to a second embodiment, the washcoat can be coated on inletand/or outlet surfaces and also within the porous structure of theporous substrate. We believe that a surface coating around a poreopening at the inlet and/or outlet surfaces, thereby narrowing the e.g.surface pore size of a bare filter substrate, promotes interaction ofthe gas phase including PM without substantially restricting the porevolume, so not giving rise to significant increases in back pressure.That is, the pores at a surface of the porous structure comprise a poreopening and the washcoat causes a narrowing of substantially all thepore openings. A schematic representation of the second embodiment isshown in FIG. 2B.

Methods of making a filter according to the second embodiment caninvolve appropriate formulation of the washcoat known to the personskilled in the art including adjusting viscosity and surface wettingcharacteristics and application of an appropriate vacuum followingcoating of the porous substrate (see also WO 99/47260).

In the first and second embodiments, wherein at least part of thewashcoat is coated on inlet and/or outlet surfaces of the poroussubstrate, the washcoat can be coated on the inlet surfaces, the outletsurfaces or on both the inlet and the outlet surfaces. Additionallyeither one or both of the inlet and outlet surfaces can include aplurality of washcoat layers, wherein each washcoat layer within theplurality of layers can be the same or different, e.g. the mean poresize in a first layer can be different from that of a second layer. Inembodiments, washcoat intended for coating on outlet surfaces is notnecessarily the same as for inlet surfaces.

Where both inlet and outlet surfaces are coated, the washcoatformulations can be the same or different. Where both the inlet and theoutlet surfaces are washcoated, the mean pore size of washcoat on theinlet surfaces can be different from the mean pore size of washcoat onthe outlet surfaces. For example, the mean pore size of washcoat on theinlet surfaces can be less than the mean pore size of washcoat on theoutlet surfaces. In the latter case, a mean pore size of washcoat on theoutlet surfaces can be greater than a mean pore size of the poroussubstrate.

Whilst it is possible for the mean pore size of a washcoat applied toinlet surfaces to be greater than the mean pore size of the poroussubstrate, it is advantageous to have washcoat having smaller pores thanthe porous substrate in washcoat on inlet surfaces to prevent or reduceany combustion ash or debris entering the porous structure.

According to a third embodiment, the washcoat sits substantially within,i.e. permeates, the porous structure of the porous substrate. Aschematic representation of this third embodiment is shown in FIG. 2C.Methods of making a filter according to the third embodiment include theappropriate formulation of the washcoat known to the person skilled inthe art including viscosity adjustment, selection of low wettingcharacteristics and application of an appropriate vacuum followingwashcoating of the porous substrate (see also WO 99/47260).Alternatively, the porous substrate can be soaked in an appropriatesolution of salts and the resulting product dried and calcined.

EP 1663458 discloses a SCR filter, wherein the filter is a wallflowmonolith and wherein an SCR catalyst composition permeates walls of thewallflow monolith. The specification discloses generally that the wallsof the wallflow filter can contain thereon or therein (i.e. not both)one or more catalytic materials. According to the disclosure,“permeate”, when used to describe the dispersion of a catalyst slurry onthe wallflow monolith substrate, means the catalyst composition isdispersed throughout the wall of the substrate.

In the second and third embodiments, wherein at least part of thewashcoat is in the porous structure, a size, e.g. a mean size, of thesolid washcoat particles can be less than the mean pore size of theporous filter substrate for example in the range 0.1 to 20 μm, such as 1to 18 μm, 1 to 16 μm, 2 to 15 μm or 3 to 12 μm. In particularembodiments, the abovementioned size of the solid washcoat particles isa D90 instead of a mean size.

In further particular embodiments, the surface porosity of the washcoatis increased by including voids therein. Exhaust gas catalysts havingsuch features are disclosed, e.g. in our WO 2006/040842 and WO2007/116881.

By “voids” in the washcoat layer herein, we mean that a space exists inthe layer defined by solid washcoat material. Voids can include anyvacancy, fine pore, tunnel-state (cylinder, prismatic column), slitetc., and can be introduced by including in a washcoat composition forcoating on the filter substrate a material that is combusted duringcalcination of a coated filter substrate, e.g. chopped cotton ormaterials to give rise to pores made by formation of gas ondecomposition or combustion. Where voids are present, voids aredifferent from, and therefore should not be counted towardsdetermination of, the average interparticle pore size of the porouswashcoat.

The average void ratio of the washcoat can be from 5-80%, whereas theaverage diameter of the voids can be from 0.2 to 500 μm, such as 10 to250 μm.

The washcoat for use in the filter of the invention is preferably acatalytic washcoat, and in embodiments is selected from the groupconsisting of a hydrocarbon trap, a three-way catalyst (TWC), a NO_(x)absorber, an oxidation catalyst, a selective catalytic reduction (SCR)catalyst, a lean NO_(x) catalyst and combinations of any two or morethereof. For example, in preferred embodiments, inlet surfaces arecoated with a TWC washcoat or NO_(x) absorber composition and the outletsurfaces are coated with SCR washcoat. In this arrangement, intermittentrich running of the engine, e.g. to regenerate the NO_(x) absorptioncapacity of the NO_(x) absorber, can generate ammonia in situ on the TWCor NO_(x) absorber for use in reducing NO_(x) on SCR catalyst on theoutlet surfaces. Similarly, an oxidation catalyst can includehydrocarbon trap functionality. In one embodiment, the inlet surfacesare not coated with SCR catalyst.

The catalytic washcoat, such as the TWC, NO_(x) absorber, oxidationcatalyst, hydrocarbon trap and the lean NO_(x) catalyst, can contain oneor more platinum group metals, particularly those selected from thegroup consisting of platinum, palladium and rhodium.

TWCs are intended to catalyse three simultaneous reactions: (i)oxidation of carbon monoxide to carbon dioxide, (ii) oxidation ofunburned hydrocarbons to carbon dioxide and water; and (iii) reductionof nitrogen oxides to nitrogen and oxygen. These three reactions occurmost efficiently when the TWC receives exhaust from an engine running ator about the stoichiometric point. As is well known in the art, thequantity of carbon monoxide (CO), unburned hydrocarbons (HC) andnitrogen oxides (NO_(x)) emitted when gasoline fuel is combusted in apositive ignition (e.g. spark-ignited) internal combustion engine isinfluenced predominantly by the air-to-fuel ratio in the combustioncylinder. An exhaust gas having a stoichiometrically balancedcomposition is one in which the concentrations of oxidising gases(NO_(x) and O₂) and reducing gases (HC and CO) are substantiallymatched. The air-to-fuel ratio that produces the stoichiometricallybalanced exhaust gas composition is typically given as 14.7:1.

Theoretically, it should be possible to achieve complete conversion ofO₂, NO_(x), CO and HC in a stoichiometrically balanced exhaust gascomposition to CO₂, H₂O and N₂ and this is the duty of the three-waycatalyst. Ideally, therefore, the engine should be operated in such away that the air-to-fuel ratio of the combustion mixture produces thestoichiometrically balanced exhaust gas composition.

A way of defining the compositional balance between oxidising gases andreducing gases of the exhaust gas is the lambda (λ) value of the exhaustgas, which can be defined according to equation (1) as:Actual engine air-to-fuel ratio/Stoichiometric engine air-to-fuelratio,  (1)wherein a lambda value of 1 represents a stoichiometrically balanced (orstoichiometric) exhaust gas composition, wherein a lambda value of >1represents an excess of O₂ and NO_(x) and the composition is describedas “lean” and wherein a lambda value of <1 represents an excess of HCand CO and the composition is described as “rich”. It is also common inthe art to refer to the air-to-fuel ratio at which the engine operatesas “stoichiometric”, “lean” or “rich”, depending on the exhaust gascomposition which the air-to-fuel ratio generates: hencestoichiometrically-operated gasoline engine or lean-burn gasolineengine.

It should be appreciated that the reduction of NO_(x) to N₂ using a TWCis less efficient when the exhaust gas composition is lean ofstoichiometric. Equally, the TWC is less able to oxidise CO and HC whenthe exhaust gas composition is rich. The challenge, therefore, is tomaintain the composition of the exhaust gas flowing into the TWC at asclose to the stoichiometric composition as possible.

Of course, when the engine is in steady state it is relatively easy toensure that the air-to-fuel ratio is stoichiometric. However, when theengine is used to propel a vehicle, the quantity of fuel requiredchanges transiently depending upon the load demand placed on the engineby the driver. This makes controlling the air-to-fuel ratio so that astoichiometric exhaust gas is generated for three-way conversionparticularly difficult. In practice, the air-to-fuel ratio is controlledby an engine control unit, which receives information about the exhaustgas composition from an exhaust gas oxygen (EGO) (or lambda) sensor: aso-called closed loop feedback system. A feature of such a system isthat the air-to-fuel ratio oscillates (or perturbates) between slightlyrich of the stoichiometric (or control set) point and slightly lean,because there is a time lag associated with adjusting air-to-fuel ratio.This perturbation is characterised by the amplitude of the air-to-fuelratio and the response frequency (Hz).

The active components in a typical TWC comprise one or both of platinumand palladium in combination with rhodium, or even palladium only (norhodium), supported on a high surface area oxide, and an oxygen storagecomponent.

When the exhaust gas composition is slightly rich of the set point,there is a need for a small amount of oxygen to consume the unreacted COand HC, i.e. to make the reaction more stoichiometric. Conversely, whenthe exhaust gas goes slightly lean, the excess oxygen needs to beconsumed. This was achieved by the development of the oxygen storagecomponent that liberates or absorbs oxygen during the perturbations. Themost commonly used oxygen storage component (OSC) in modern TWCs iscerium oxide (CeO₂) or a mixed oxide containing cerium, e.g. a Ce/Zrmixed oxide.

NO_(x) absorber catalysts (NACs) are known e.g. from U.S. Pat. No.5,473,887 and are designed to adsorb nitrogen oxides (NO_(x)) from leanexhaust gas (lambda >1) and to desorb the NO_(x) when the oxygenconcentration in the exhaust gas is decreased. Desorbed NO_(x) may bereduced to N₂ with a suitable reductant, e.g. gasoline fuel, promoted bya catalyst component, such as rhodium, of the NAC itself or locateddownstream of the NAC. In practice, control of oxygen concentration canbe adjusted to a desired redox composition intermittently in response toa calculated remaining NO_(x) adsorption capacity of the NAC, e.g.richer than normal engine running operation (but still lean ofstoichiometric or lambda=1 composition), stoichiometric or rich ofstoichiometric (lambda <1). The oxygen concentration can be adjusted bya number of means, e.g. throttling, injection of additional hydrocarbonfuel into an engine cylinder such as during the exhaust stroke orinjecting hydrocarbon fuel directly into exhaust gas downstream of anengine manifold.

A typical NAC formulation includes a catalytic oxidation component, suchas platinum, a significant quantity, i.e. substantially more than isrequired for use as a promoter such as a promoter in a TWC, of aNO_(x)-storage component, such as barium, and a reduction catalyst, e.g.rhodium. One mechanism commonly given for NO_(x)-storage from a leanexhaust gas for this formulation is:NO+½O₂→NO₂  (2); andBaO+NO₂+½O₂→Ba(NO₃)₂  (3);wherein in reaction (2), the nitric oxide reacts with oxygen on activeoxidation sites on the platinum to form NO₂. Reaction (3) involvesadsorption of the NO₂ by the storage material in the form of aninorganic nitrate.

At lower oxygen concentrations and/or at elevated temperatures, thenitrate species become thermodynamically unstable and decompose,producing NO or NO₂ according to reaction (4) below. In the presence ofa suitable reductant, these nitrogen oxides are subsequently reduced bycarbon monoxide, hydrogen and hydrocarbons to N₂, which can take placeover the reduction catalyst (see reaction (5)).Ba(NO₃)₂→BaO+2NO+ 3/2O₂ or Ba(NO₃)₂→BaO+2NO₂+½O₂  (4); andNO+CO→½N₂+CO₂  (5);(Other reactions include Ba(NO₃)₂+8H₂→BaO+2NH₃+5H₂O followed byNH₃+NO_(x)→N₂+yH₂O or 2NH₃+2O₂+CO→N₂+3H₂O+CO₂ etc.).

In the reactions of (2)-(5) above, the reactive barium species is givenas the oxide. However, it is understood that in the presence of air mostof the barium is in the form of the carbonate or possibly the hydroxide.The skilled person can adapt the above reaction schemes accordingly forspecies of barium other than the oxide and sequence of catalyticcoatings in the exhaust stream.

Oxidation catalysts promote the oxidation of carbon monoxide to carbondioxide and unburned hydrocarbons to carbon dioxide to water. Typicaloxidation catalysts include platinum and/or palladium on a high surfacearea support.

Hydrocarbon traps typically include molecular sieves and may also becatalysed e.g. with a platinum group metal such as platinum or acombination of both platinum and palladium.

SCR catalysts can be selected from the group consisting of at least oneof Cu, Hf, La, Au, In, V, lanthanides and Group VIII transition metals,such as Fe, supported on a refractory oxide or molecular sieve. Suitablerefractory oxides include Al₂O₃, TiO₂, CeO₂, SiO₂, ZrO₂ and mixed oxidescontaining two or more thereof. The non-zeolite catalyst can alsoinclude tungsten oxide, e.g. V₂O₅/WO₃/TiO₂.

Lean NO_(x) catalysts, sometimes also called hydrocarbon-SCR catalysts,DeNO_(x) catalysts or even non-selective catalytic reduction catalysts,include Pt/Al₂O₃, Cu-, Pt-, Fe-, Co- or Ir-exchanged ZSM-5, protonatedzeolites such as H-ZSM-5 or H—Y zeolites, perovskites and Ag/Al₂O₃. Inselective catalytic reduction (SCR) by hydrocarbons (HC), HC react withNOx, rather than with O₂, to form nitrogen, CO₂ and water according toequation (6):{HC}+NOx→N₂+CO₂+H₂O  (6)

The competitive, non-selective reaction with oxygen is given by Equation(7):{HC}+O₂→CO₂+H₂O  (7)

Therefore, good HC-SCR catalysts are more selective for reaction (6)than reaction (7).

In particular embodiments, the washcoat comprises at least one molecularsieve, such as an aluminosilicate zeolite or a SAPO, for trappingpositive ignition PM. The at least one molecular sieve can be a small, amedium or a large pore molecular sieve, for example. By “small poremolecular sieve” herein we mean molecular sieves containing a maximumring size of 8, such as CHA; by “medium pore molecular sieve” herein wemean a molecular sieve containing a maximum ring size of 10, such asZSM-5; and by “large pore molecular sieve” herein we mean a molecularsieve having a maximum ring size of 12, such as beta. Small poremolecular sieves are potentially advantageous for use in SCRcatalysts—see for example WO 2008/132452.

Particular molecular sieves with application in the present inventionare selected from the group consisting of AEI, ZSM-5, ZSM-20, ERIincluding ZSM-34, mordenite, ferrierite, BEA including Beta, Y, CHA, LEVincluding Nu-3, MCM-22 and EU-1.

In embodiments, the molecular sieves can be un-metallised or metallisedwith at least one metal selected from the group consisting of groups IB,IIB, IIIA, IIIB, VB, VIB, VIIB and VIII of the periodic table. Wheremetallised, the metal can be selected from the group consisting of Cr,Co, Cu, Fe, Hf, La, Ce, In, V, Mn, Ni, Zn, Ga and the precious metalsAg, Au, Pt, Pd and Rh. Such metallised molecular sieves can be used in aprocess for selectively catalysing the reduction of nitrogen oxides inpositive ignition exhaust gas using a reductant. By “metallised” hereinwe mean to include molecular sieves including one or more metalsincorporated into a framework of the molecular sieve e.g. Fein-framework Beta and Cu in-framework CHA. As mentioned above, where thereductant is a hydrocarbon, the process is sometimes called “hydrocarbonselective catalytic reduction (HC-SCR)”, “lean NO_(x) catalysis” or“DeNO_(x) catalysis”, and particular metals for this application includeCu, Pt, Mn, Fe, Co, Ni, Zn, Ag, Ce, Ga. Hydrocarbon reductant can eitherbe introduced into exhaust gas by engine management techniques, e.g.late post injection or early post injection (so-called “afterinjection”).

Where the reductant is a nitrogenous reductant (so-called “NH₃—SCR”),metals of particular interest are selected from the group consisting ofCe, Fe and Cu. Suitable nitrogenous reductants include ammonia. Ammoniacan be generated in situ e.g. during rich regeneration of a NAC disposedupstream of the filter or by contacting a TWC with engine-derived richexhaust gas (see the alternatives to reactions (4) and (5) hereinabove).Alternatively, the nitrogenous reductant or a precursor thereof can beinjected directly into the exhaust gas. Suitable precursors includeammonium formate, urea and ammonium carbamate. Decomposition of theprecursor to ammonia and other by-products can be by hydrothermal orcatalytic hydrolysis.

The cell density of diesel wallflow filters in practical use can bedifferent from wallflow filters for use in the present invention in thatthe cell density of diesel wallflow filters is generally 300 cells persquare inch (cpsi) or less, e.g. 100 or 200 cpsi, so that the relativelylarger diesel PM components can enter inlet channels of the filterwithout becoming impacted on the solid frontal area of the dieselparticulate filter, thereby caking and fouling access to the openchannels, whereas wallflow filters for use in the present invention canbe up to 300 cpsi or greater, such as 350 cpsi, 400 cpsi, 600 cpsi, 900cpsi or even 1200 cpsi.

An advantage of using higher cell densities is that the filter can havea reduced cross-section, e.g. diameter, than diesel particulate filters,which is a useful practical advantage that increases design options forlocating exhaust systems on a vehicle.

According to a further aspect, the invention provides an exhaust systemfor a positive ignition engine, which system comprising a filteraccording to the invention. Positive ignition engines for use in thisaspect of the invention can be fuelled by gasoline fuel, gasoline fuelblended with oxygenates including methanol and/or ethanol, liquidpetroleum gas or compressed natural gas.

In one embodiment, the exhaust system comprises means for injecting areductant fluid, e.g. a hydrocarbon or nitrogenous reductant or aprecursor thereof, into exhaust gas upstream of the filter. In aparticular embodiment, the reductant fluid is a nitrogenous compound.

In a particular embodiment, the injector and filter are both locateddownstream of a TWC.

In another aspect, the invention provides a positive ignition enginecomprising an exhaust system according to the invention and to a vehiclecomprising such a positive ignition engine. In a preferred embodiment,the positive ignition engine is a direct injection positive ignitionengine.

In a further aspect, the invention provides a method of trappingparticulate matter (PM) from exhaust gas emitted from a positiveignition engine by depth filtration, which method comprising contactingexhaust gas containing the PM with a filter comprising a poroussubstrate having inlet and outlet surfaces, wherein the inlet surfacesare separated from the outlet surfaces by a porous structure containingpores of a first mean pore size, wherein the porous substrate is coatedwith a washcoat comprising a plurality of solid particles wherein theporous structure of the washcoated porous substrate contains pores of asecond mean pore size, and wherein the second mean pore size is lessthan the first mean pore size.

FIGS. 2A-C show a cross-section through a porous filter substrate 10comprising a surface pore 12. FIG. 2A shows a first embodiment,featuring a porous surface washcoat layer 14 comprised of solid washcoatparticles, the spaces between which particles define pores(interparticle pores). It can be seen that the washcoat layer 14substantially covers the pore 12 of the porous structure and that a meanpore size of the interparticle pores 16 is less than the mean pore size12 of the porous filter substrate 10.

FIG. 2B shows a second embodiment comprising a washcoat that is coatedon an inlet surface 16 and additionally within a porous structure 12 ofthe porous substrate 10. It can be seen that the washcoat layer 14causes a narrowing of a pore openings of surface pore 12, such that amean pore size 18 of the coated porous substrate is less than the meanpore size 12 of the porous filter substrate 10.

FIG. 2C shows a third embodiment wherein the washcoat 14 sitssubstantially within, i.e. permeates, the porous 12 structure of theporous substrate 10.

FIG. 3 shows an illustration of a graph relating pore size to porenumber for a porous filter substrate 20, a porous washcoat layer 22 anda porous diesel filter substrate including a surface washcoat layer 24.It can be seen that the filter substrate has a mean pore size of theorder of about 15 μm. The washcoat layer has a bimodal distributioncomprised of intraparticle pores 22A (at the nanometer end of the range)and interparticle pores 22B towards the micrometer end of the scale. Itcan also be seen that by coating the porous filter substrate with awashcoat according to the invention that the pore distribution of thebare filter substrate is shifted in the direction of the interparticlewashcoat pore size (see arrow).

FIG. 4 sets out a matrix showing preliminary results for a washcoatloading study for a three-way catalyst washcoat on three wallflowfilters having different mean pore sizes. In conclusion, there is a bandof acceptable backpressure and filtration starting with a combination of13 μm mean pore size wallflow filter and relatively low washcoat loading(0.4 g in⁻³) through the 20 μm and 13 μm pore size substrates having 0.8g in⁻³ to the 1.6 and 2.4 g in⁻³ loadings on the 38 μm and 20 μm meanpore size substrates.

However, overlying this matrix for three-way catalyst use is thatwashcoat loadings of ≧1.6 g in⁻³ are preferred for acceptable three-waycatalyst activity in a stand-alone product. The invention allows acombination of sufficient three-way catalyst activity and PM filtrationto be achieved without a significant increase in backpressure. Increasedwashcoat loadings on lower mean pore size wallflow filter substrates canonly be used in applications that can tolerate increased backpressure.With reference to FIG. 4, whilst in certain applications wherebackpressure increases can be tolerated, a 13 μm mean pore size wallflowfilter substrate can be used in combination with ≧1.6 g in⁻³ washcoatloading, we presently prefer to use a mean pore size of ≧20 μm for ≧1.6g in⁻³ loadings to achieve a desirable balance between catalystactivity, filtration and backpressure. A benefit of the invention isthat a state-of-the-art three-way catalyst comprising a flow-throughmonolith substrate that is typically located on a vehicle in either anunderfloor or close-coupled location can be replaced with a filteraccording to the invention to provide sufficient three-way activity tomeet legislative requirements for gaseous HC, CO and NOx emissions,while also meeting particle number standards as required by e.g. Euro 6standards.

The filter according to the invention could obviously be used incombination with other exhaust system aftertreatment components toprovide a full exhaust system aftertreatment apparatus, e.g. a lowthermal mass TWC upstream of the filter and/or downstream catalyticelements, e.g. NO_(x) trap or SCR catalyst, according to specificrequirements. So, in vehicular positive ignition applications producingrelatively cool on-drive cycle exhaust gas temperatures, we contemplateusing a low thermal mass TWC disposed upstream of the filter accordingto the invention. For vehicular lean-burn positive ignitionapplications, we envisage using a filter according to the inventionupstream or downstream of a NO_(x) trap. In vehicularstoichiometrically-operated positive ignition engines, we believe thatthe filter according to the present invention can be used as astandalone catalytic exhaust system aftertreatment component. That is,in certain applications the filter according to the present invention isadjacent and in direct fluid communication with the engine withoutintervening catalysts therebetween; and/or an exit to atmosphere from anexhaust gas aftertreatment system is adjacent to and in direct fluidcommunication with the filter according to the present invention withoutintervening catalysts therebetween.

An additional requirement of a TWC is a need to provide a diagnosisfunction for its useful life, so called “on-board diagnostics” or OBD. Aproblem in OBD arises where there is insufficient oxygen storagecapacity in the TWC, because OBD processes for TWCs use remaining oxygenstorage capacity to diagnose remaining catalyst function. However, ifinsufficient washcoat is loaded on the filter such as in the specificExamples disclosed in US 2009/0193796 and WO 2009/043390, there may notbe enough OSC present to provide an accurate OSC “delta” for OBDpurposes. Since the present invention enables washcoat loadingsapproaching current state-of-the-art TWCs, the filters for use in thepresent invention can be used with advantage in current OBD processes.

EXAMPLES

In order that the invention may be more fully understood the followingExamples are provided by way of illustration only. The washcoat loadingsquoted in the Examples were obtained using the method disclosed in WO99/47260 described hereinabove by coating half of the washcoat from oneend and the remaining half of the washcoat from the other end, i.e. theentire washcoat was not coated only on the inlet or outlet channels ofthe filter, but on both the inlet and outlet channels of the filter.

Example 1

Two three-way catalyst (TWC) coatings were prepared at a washcoatloading of 2.4 g/in³ and a precious metal loading of 85 g/ft³ (Pd:Rh16:1); one was milled to a small particle size (d90<5 μm) that would beexpected to pass into the pore structure of a wallflow filter(“in-wall”), while the other was less milled (d90<17 μm) so that itwould be expected preferentially to locate more at the surface of awallflow filter wall (“on-wall”). The coatings were applied to 4.66×4.5inch 300 cells per square inch cordierite wallflow filter substrateshaving 12 thousandths of an inch wall thickness (“300/12”) with anominal average pore size of 20 micrometers (hereinafter “microns”) (62%porosity). Each filter was hydrothermally oven-aged at 980° C. for 4hours and installed in a close-coupled position on a Euro 5 passengercar with a 1.4 L direct injection gasoline engine. Each filter wasevaluated over a minimum of three MVEG-B drive cycles, measuring thereduction in particle number emissions relative to a reference catalyst,wherein the filter was exchanged for a TWC coated onto a flowthroughsubstrate monolith at the same washcoat and precious metal loadings—andthe backpressure differential was determined between sensors mountedupstream and downstream of the filter (or reference catalyst).

In Europe, since the year 2000 (Euro 3 emission standard) emissions aretested over the New European Driving Cycle (NEDC). This consists of fourrepeats of the previous ECE 15 driving cycle plus one Extra UrbanDriving Cycle (EUDC) with no 40 second warm-up period before beginningemission sampling. This modified cold start test is also referred to asthe “MVEG-B” drive cycle. All emissions are expressed in g/km.

The Euro 5/6 implementing legislation introduces a new PM mass emissionmeasurement method developed by the UN/ECE Particulate MeasurementProgramme (PMP) which adjusts the PM mass emission limits to account fordifferences in results using old and the new methods. The Euro 5/6legislation also introduces a particle number emission limit (PMPmethod), in addition to the mass-based limits.

The results in Table 1 demonstrate that the filters prepared with thelarger particle size “on-wall” washcoat have significantly improvedparticle number reduction than the filters prepared with the smallerparticle size “in-wall” washcoat, with a small, but acceptable, increasein peak backpressure.

TABLE 1 Effect of washcoat location within filter on particle numberreduction and backpressure (BP) % PN Average BP Peak BP reduction (mbar)on 70 kph (mbar) during vs. flow cruise of any one Sample filterWashcoat through MVEG-B drive MVEG-B drive properties type referencecycle cycle 20 μm, 62% “In-wall” 75 14.3 73.5 20 μm, 62% “On-wall” 8316.2 104.2

Example 2

5.66×3 inch cordierite wallflow filter substrates with a cell density of300 cells per square inch and a wall thickness of 12 thousandths of aninch (approximately 0.3 mm) were coated with a three-way catalyst (TWC)coating at a washcoat loading of 0.8 g/in³ and a palladium loading of 80g/ft³. Three pore structures were compared: a nominal average pore sizeof 38 microns at 65% porosity, a nominal average pore size of 20 micronsat 62% porosity and a nominal average pore size of 15 microns at 52%porosity. Each filter was hydrothermally oven-aged at 980° C. for 4hours and installed in the underfloor position on a Euro 4 passenger carwith a 1.4 L direct injection gasoline engine, with a fully formulatedthree-way catalyst coated on a flowthrough substrate monolith located inthe close-coupled position, i.e. upstream of the filter. Each filter wasevaluated over a minimum of three MVEG-B drive cycles, measuring thereduction in particle number emissions relative to a reference system,wherein the underfloor filter was exchanged for a TWC coated on aflowthrough substrate monolith at identical washcoat and palladiumloadings and the backpressure differential was determined betweensensors mounted upstream of the close-coupled TWC and downstream of thefilter (or reference catalyst). The peak backpressure results given inTable 2 are the backpressure reading on the third repeat of the MVEG-Bcycle.

The results in Table 2 demonstrate that the 38 micron filter hadsignificantly lower levels of particle number removal (insufficient forthis vehicle application), albeit with the lowest backpressure. The 20micron filter gave acceptable levels of particle number reduction with amoderate increase in backpressure. The 15 micron filter was mosteffective at reducing particle number emissions but had significantlyhigher backpressure than the 20 micron filter embodiment.

TABLE 2 Comparison of particle number reduction and backpressure (BP)for different pore size filters Average BP Peak BP (mbar) on 70 kph(mbar) % PN reduction cruise of during third Sample filter vs. flowthrough third MVEG-B MVEG-B properties reference drive cycle drive cycle38 μm, 65% 18 7.5 52.5 20 μm, 62% 85 12.1 68.9 15 μm, 52% 92 18.8 97.5

Example 3

4.66×4.5 inch, 300/12 cordierite wallflow filter substrates with anominal average pore size of 20 microns and porosity of 62% were coatedwith a three-way catalyst coating at washcoat loadings of 0.8, 1.6 and2.4 g/in³ respectively. Each sample had a precious metal loading of 85g/ft³ (Pd:Rh 16:1). Each filter was hydrothermally oven-aged at 980° C.for 4 hours and installed in a close-coupled position on a Euro 4passenger car with a 1.4 L direct injection gasoline engine. Each filterwas evaluated over a minimum of three MVEG-B drive cycles, measuring thereduction in particle number emissions relative to a reference catalyst,wherein the close-coupled filter was exchanged for a TWC coated on aflowthrough substrate monolith at an identical washcoat and preciousmetal loading, the backpressure differential and the conversionefficiency for gaseous HC, CO and NO_(x) emissions were determinedbetween sensors mounted upstream and downstream of the filter (orreference catalyst). Only non-methane hydrocarbons (NMHC) conversion isreported in Table 3 (the NMHC for Euro 6 is 68 mg/km within a totalhydrocarbon emission limit of 100 mg/km).

The results in Table 3 demonstrate that the filter prepared with awashcoat loading of 0.8 g/in³ had significantly lower levels of particlenumber removal and the lowest NMHC conversion efficiency. Such TWCperformance would not be sufficient to meet Euro 6 gaseous emissionslimits for a typical passenger car. Increasing the washcoat loading to1.6 and 2.4 g/in³ gave greater reductions in particle number emissions,albeit at increasing, but acceptable, backpressure. TWC activity (asrepresented in Table 3 by NMHC performance) was also significantlyimproved with the higher washcoat loadings.

TABLE 3 Comparison of particle number reduction, backpressure (BP) andTWC activity at different washcoat loadings % PN Average BP Peak BPreduction (mbar) on 70 kph (mbar) during % of Euro 6 Sample vs. flowcruise of any one NMHC washcoat through MVEG-B MVEG-B engineeringloading reference drive cycle drive cycle target^(†) 0.8 53 7.7 51 1101.6 63 10.1 65 88 2.4 67 18.7 100 81 ^(†)The “engineering target” isoften used by the vehicle manufacturers and represents a percentage ofthe legislated emissions. For the purposes of these Examples, we haveused an engineering target of 80%. Since the Euro 6 NMHC standard is 68mg/km, the engineering target is 54 mg/km. The calculated percentage ofthis number is used to assess the reduction in NMHC results achievedover the MVEG-B drive cycle. This gives values above and below 100% thatrelate well to the acceptable three-way catalyst activity.

Example 4

4.66×4.5 inch, 300/12 cordierite wallflow filter substrates with a celldensity of 300 cells per square inch and a wall thickness ofapproximately 0.3 mm were coated with a three-way catalyst coating at awashcoat loading of 1.6 g/in³ and a precious metal loading of 85 g/ft³(Pd:Rh 16:1). Two pore structures were compared: a nominal average poresize of 38 microns at 65% porosity and a nominal average pore size of 20microns at 62% porosity. A smaller pore sample was not evaluatedbecause, from the results obtained from the Example 2, the backpressurewas expected to be too great for the Euro 4 passenger car in this test.Each filter was hydrothermally oven aged at 980° C. for 4 hours andinstalled in a close-coupled position on a Euro 4 passenger car with a1.4 L direct injection gasoline engine. Each filter was evaluated over aminimum of three MVEG-B drive cycles, measuring the reduction inparticle number emissions relative to a reference catalyst, wherein theclose-coupled filter was exchanged for a TWC coated on a flowthroughsubstrate monolith at an identical washcoat and precious metal loading,the backpressure differential and the conversion efficiency for gaseousHC, CO and NO_(x) emissions were determined between sensors mountedupstream and downstream of the filter (or reference catalyst). Onlynon-methane hydrocarbons (NMHC) conversion is reported in Table 4.

The results in Table 4 demonstrate that the 38 micron filter hadsignificantly lower levels of particle number removal (insufficient forthis vehicle application) and lower backpressure, which may beacceptable in other vehicle applications. The 20 micron filter gave goodlevels of particle number reduction with a moderate increase inbackpressure. Both samples had good TWC activity at a washcoat loadingof 1.6 g/in³.

TABLE 4 Comparison of particle number reduction, backpressure (BP) andTWC activity for different pore size filters Average BP Peak BP % PN(mbar) on 70 (mbar) during % of Euro 6 Sample reduction vs. kph cruiseof any one NMHC filter flow through MVEG-B MVEG-B engineering propertiesreference drive cycle drive cycle target^(†) 38 μm, 65% 34 5.9 43.4 8820 μm, 62% 63 10.1 65 88 ^(†)See footnote to Table 3.

Example 5

4.66×4.5 inch, 300/12 cordierite wallflow filter substrates with a celldensity of 300 cells per square inch and a wall thickness ofapproximately 0.3 mm were coated with a three-way catalyst coating at awashcoat loading of 2.4 g/in³ and a precious metal loading of 85 g/ft³(Pd:Rh 16:1). Two pore structures were compared: a nominal average poresize of 38 microns at 65% porosity and a nominal average pore size of 20microns at 62% porosity. A smaller pore sample was not evaluatedbecause, from the results obtained from the Example 2, the backpressurewas expected to be too great for the Euro 5 passenger car in this test.Each filter was hydrothermally oven-aged at 980° C. for 4 hours andinstalled in a close-coupled position on a Euro 5 passenger car with a1.4 L direct injection gasoline engine. The filters were evaluated overa minimum of three MVEG-B drive cycles, measuring the reduction inparticle number emissions relative to a reference catalyst, wherein theclose-coupled filter was exchanged for a TWC coated on a flowthroughsubstrate monolith at an identical washcoat and precious metal loading,the backpressure differential and the conversion efficiency for gaseousHC, CO and NO_(x) emissions were determined between sensors mountedupstream and downstream of the filter (or reference catalyst). Onlynon-methane hydrocarbons (NMHC) conversion is reported in Table 5.

The results in Table 5 demonstrate that the 38 micron filter hadsignificantly lower levels of particle number removal (borderline forthis vehicle application) and lower backpressure than the 20 micronfilter embodiment. The 20 micron filter gave good levels of particlenumber reduction with a moderate increase in backpressure. Both sampleshad good TWC activity at a washcoat loading of 2.4 g/in³. Both samplesdisplayed greater particle number reduction and increased backpressurecompared to the 1.6 g/in³ samples described in Example 4.

TABLE 5 Comparison of particle number reduction, backpressure (BP) andTWC activity for different pore size filters % PN Average BP Peak BPreduction (mbar) on 70 (mbar) during % of Euro 6 Sample vs. flow kphcruise of any one NMHC filter through MVEG-B MVEG-B engineeringproperties reference drive cycle drive cycle target^(†) 38 μm, 65% 507.4 44.1 44 20 μm, 62% 75 14.3 73.5 53 ^(†)See footnote to Table 3.

Example 6

118×60 mm, 360 cells per square inch cordierite wallflow filtersubstrates having 5 thousandths of an inch cell wall thickness (360/5)with a nominal average pore size of 13 microns and porosity of 48% werecoated with a three-way catalyst coating at washcoat loadings of 0.4 and0.8 g/in³. Each sample had a precious metal loading of 85 g/ft³ (Pd:Rh16:1). Higher washcoat loadings were not evaluated because the resultingbackpressure was expected to be too great for the Euro 4 passenger carin this test. A fresh (i.e. un-aged) filter was installed in aclose-coupled position on a Euro 4 passenger car with a 1.4 L directinjection gasoline engine. Each filter was evaluated over a minimum ofthree MVEG-B drive cycles, measuring the reduction in particle numberemissions relative to a reference catalyst, wherein the close-coupledfilter was exchanged for a TWC coated on a flowthrough substratemonolith at an identical washcoat and precious metal loading and thebackpressure differential and the conversion efficiency for gaseous HC,CO and NO_(x) emissions were determined between sensors mounted upstreamand downstream of the filter (or reference catalyst). Only non-methanehydrocarbons (NMHC) conversion is reported in Table 6.

The results in Table 6 demonstrate that the 13 micron filter preparedwith a washcoat loading of 0.8 g/in³ gave moderate levels of particlenumber removal (borderline for this vehicle application) but hadextremely high backpressure. Reducing the washcoat loading to 0.4 g/in³gave more acceptable backpressure but a smaller reduction in particlenumber emissions. Such low washcoat levels would not be expected to givesufficient three-way catalyst activity to meet Euro 6 emissionstandards.

TABLE 6 Comparison of particle number reduction and backpressure (BP) atdifferent washcoat loadings Average BP Peak BP (mbar) Sample % PNreduction vs. (mbar) on 70 kph during any one washcoat flow throughcruise of MVEG-B MVEG-B drive loading reference drive cycle cycle 0.4 5011.3 78.4 0.8 54 45.2 211.8

Example 7

A Euro 5 passenger car with a 2.0 L direct injection gasoline engineequipped with a fully formulated three-way catalyst coated on aflowthrough substrate monolith in the close-coupled position was testedover the MVEG-B and FTP (Federal Test Procedure) 75 drive cycles. Thenumber of particles emitted over the MVEG-B drive cycle was measuredaccording to the PMP methodology. The mass of particulate matter emittedover the FTP 75 drive cycle was measured following standard protocols. A125×120 mm, 300/12 cordierite wallflow filter with a nominal averagepore size of 12 microns and porosity of 55% coated with a three-waycatalyst coating at a washcoat loading of 0.8 g/in³ and a precious metalloading of 20 g/ft³ (Pd:Rh 3:1) was then fitted in the underfloorposition, i.e. downstream of the flowthrough substrate monolith.Particulate mass and number emissions measurements were repeated.

The results in Table 7 demonstrate that fitment of the additional coatedfilter reduced particle number emissions over the MVEG-B cycle by ˜99%and reduced the particulate mass emitted over the FTP 75 cycle by ˜75%relative to the flowthrough TWC-only system. Depending what CARB PMemission standard is adopted, the 2.7 mg PM/mile figure could fail thatstandard.

TABLE 7 Effect of filter fitment on particulate number and massemissions PN emissions over PM emissions over MVEG-B drive FTP 75 drivecycle Catalyst system cycle (#/km) (mg/mi) Flowthrough TWC 4.42 × 10¹²2.7 only Flowthrough TWC + 4.69 × 10¹⁰ 0.6 Coated filter

For the avoidance of any doubt, the entire contents of all prior artdocuments cited herein is incorporated herein by reference.

1. A filter for filtering particulate matter (PM) from exhaust gasemitted from a positive ignition engine, which filter comprising aporous substrate having inlet surfaces and outlet surfaces, wherein theinlet surfaces are separated from the outlet surfaces by a porousstructure containing pores of a first mean pore size, wherein the poroussubstrate is coated with a washcoat comprising a plurality of solidparticles wherein the porous structure of the washcoated poroussubstrate contains pores of a second mean pore size, and wherein thesecond mean pore size is less than the first mean pore size.
 2. A filteraccording to claim 1, wherein a first mean pore size of the porousstructure of the porous substrate is from 8 to 45 μm.
 3. A filteraccording to claim 1, wherein the washcoat loading is >0.50 g in⁻³.
 4. Afilter according to claim 3, wherein the washcoat loading is >1.00 gin⁻³.
 5. A filter according to claim 1, comprising a surface washcoat,wherein a washcoat layer substantially covers surface pores of theporous structure and the pores of the washcoated porous substrate aredefined in part by spaces between the particles (interparticle pores) inthe washcoat layer.
 6. A filter according to claim 5, wherein a meaninterparticle pore size of the washcoat layer is from 5.0 nm to 5.0 μm.7. A filter according to claim 1, wherein a mean size of solid washcoatparticles is greater than the first mean pore size.
 8. A filteraccording to claim 7, wherein the mean size of the solid washcoatparticles is in the range 1 to 40 μm.
 9. A filter according to claim 1,wherein the pores at a surface of the porous structure comprise a poreopening and the washcoat causes a narrowing of substantially all thesurface pore openings.
 10. A filter according to claim 1, wherein thewashcoat sits substantially within the porous structure of the poroussubstrate.
 11. A filter according to claim 9, wherein a mean size ofsolid washcoat particles is less than a mean pore size of the poroussubstrate.
 12. A filter according to claim 11, wherein a mean size ofsolid washcoat particles is in the range 0.1 to 20 μm.
 13. A filteraccording to claim 11, wherein a D90 of solid washcoat particles is inthe range 0.1 to 20 μm.
 14. A filter according to claim 1, wherein thewashcoat is coated on inlet surfaces, outlet surfaces or both the inletand the outlet surfaces.
 15. A filter according to claim 14, whereinboth the inlet and the outlet surfaces are washcoated and wherein a meanpore size of washcoat on the inlet surfaces is different from a meanpore size of washcoat on the outlet surfaces.
 16. A filter according toclaim 15, wherein the mean pore size of washcoat on the inlet surfacesis less than the mean pore size of washcoat on the outlet surfaces. 17.A filter according to claim 16, wherein a mean pore size of washcoat onthe outlet surfaces is greater than a mean pore size of the poroussubstrate.
 18. A filter according to claim 1, wherein the poroussubstrate is selected from the group consisting of a ceramic wallflowfilter, a metal filter and a ceramic foam.
 19. A filter according toclaim 18, wherein the metal filter is selected from the group consistingof a sintered metal filter, a partial filter and combinations thereofand optionally comprises a wire mesh.
 20. A filter according to claim 1,wherein the washcoat is a catalytic washcoat.
 21. A filter according toclaim 20, wherein the catalytic washcoat is selected from the groupconsisting of a hydrocarbon trap, a three-way catalyst, a NO_(x)absorber, an oxidation catalyst, a selective catalytic reduction (SCR)catalyst and a lean NO_(x) catalyst.
 22. A filter according to claim 20,wherein the catalytic washcoat comprises at least one molecular sieve.23. A filter according to claim 22, wherein the at least one molecularsieve is a small, medium or large pore molecular sieve.
 24. A filteraccording to claim 22, wherein the at least one molecular sieve isselected from the group consisting of AEI, ZSM-5, ZSM-20, ERI, LEV,mordenite, BEA, Y, CHA, MCM-22 and EU-1.
 25. A filter according to claim22, wherein the molecular sieve is un-metallised or is metallised withat least one metal selected from the group consisting of groups IB, IIB,IIIA, IIIB, VB, VIB, VIIB and VIII of the periodic table.
 26. A filteraccording to claim 25, wherein the molecular sieve is metallised and themetal is selected from the group consisting of Cr, Co, Cu, Fe, Hf, La,Ce, In, V, Mn, Ni, Zn, Ga and the precious metals Ag, Au, Pt, Pd and Rh.27. A filter according to claim 26, wherein the metal is selected fromthe group consisting of Cu, Pt, Mn, Fe, Co, Ni, Zn, Ag, Ce and Ga.
 28. Afilter according to claim 27, wherein the metal is selected from thegroup consisting of Ce, Fe and Cu.
 29. An exhaust system for a positiveignition engine, which system comprising a filter according to claim 1.30. An exhaust system according to claim 29, comprising means forinjecting a reductant fluid into exhaust gas upstream of the filter. 31.An exhaust system according to claim 30, wherein the reductant fluid isa nitrogenous compound.
 32. A positive ignition engine comprising anexhaust system according to claim
 29. 33. A positive ignition engineaccording to claim 31, fuelled with a hydrocarbon fuel selected from thegroup consisting of gasoline, gasoline blended with methanol and/orethanol, liquid petroleum gas and compressed natural gas.
 34. A methodof trapping particulate matter (PM) from exhaust gas emitted from apositive ignition engine by depth filtration, which method comprisingcontacting exhaust gas containing the PM with a filter comprising aporous substrate having inlet and outlet surfaces, wherein the inletsurfaces are separated from the outlet surfaces by a porous structurecontaining pores of a first mean pore size, wherein the porous substrateis coated with a washcoat comprising a plurality of solid particleswherein the porous structure of the washcoated porous substrate containspores of a second mean pore size, and wherein the second mean pore sizeis less than the first mean pore size.